The present invention relates to an improved over-temperature alarm system for an appliance having a freezer compartment for storing perishable items.
With appliances such as refrigerators and freezers it is desirable to provide a warning indication to the user when the temperature conditions in the freezer present a risk of damage to perishable items stored therein.
Implementation of an over-temperature warning system in such appliances is complicated by the fact that during normal use the freezer compartment door is opened frequently for the insertion and removal of items. When the freezer door is opened the temperature of the air within the freezer increases rapidly toward the ambient room temperature. However, since the temperature of the items refrigerated in the compartment changes relatively slowly, such items are not adversely affected by such occasional increases in air temperature within the freezer unless such conditions exist for a sufficient time to allow the temperature of the items themselves to rise to unacceptably high temperatures. Normally once the door is closed the temperature within the freezer returns to within its normal operating limits quickly enough to prevent any damage to the refrigerated items. This recovery time varies greatly, however, and is a function of a number of factors including how long the door was open, the ambient room temperature, the number of refrigerated items present in the freezer when the door was opened and the temperature and quantity of items being added to the freezer. A second cause of temporary over-temperature conditions in freezers of the so-called frost free type is the automatic defrost cycle. During this cycle the heat generated by the defrost heater to melt frost from the evaporator coils raises the air temperature in the freezer above its normal operating limits. A satisfactory warning system must have sufficient sensitivity to alert the user to over-temperature conditions resulting from abnormal system operation quickly enough to enable the user to take precautions before damage to the refrigerated items occurs while at the same time avoiding nuisance alarms resulting from temporary over-temperature conditions which are likely to occur during normal operation.
In U.S. Pat. No. 4,407,141 Paddock recognizes this problem, noting that in conventional refrigeration apparatus in which the temperature sensor is responsive to the temperature in the fresh food compartment it is desirable to set the trip point at the relatively high temperature of 60.degree. F. even though normal operating temperature is below 32.degree. F. in order to avoid nuisance alarms. Paddock suggests that the system may be made more sensitive by employing a second sensor responsive to the room temperature which is used in combination with the internal fresh food temperature sensor to vary the trip point as a function of the room ambient temperature, allowing the trip point to be set closer to the normal temperature range for lower room temperatures.
U.S. Pat. No. 4,387,578-Paddock discloses an over-temperature alarm system for a refrigerator appliance in which the single 60.degree. F. set point is employed in combination with a timer which monitors the duration of the over-temperature condition and provides a visual warning signal only after the over-temperature condition has continued for a predetermined time such as 11/2 hours. The visual signal changes from a steady signal to a flashing signal when the temperature drops below the threshold and continues to flash until switched off by the user to alert the user that an over-temperature condition has occurred. In addition, if the condition persists for 10 hours an alert symbol is energized, warning the user that the abnormal condition has existed for a relatively long time.
Both of the Paddock approaches utilize a single set point greater than the desired operating range. Consequently, there is a range between the set point and the desired operating range within which the air temperature in the freezer may stabilize or increase so slowly that the refrigerated items become damaged before the set point is reached.
Another approach to the problem is disclosed in U.S. Pat. No. 3,343,151 to Brown et al, which teaches the use of a temperature sensitive device having substantially the same time-temperature constant as the product or article being refrigerated. When disposed in the same environment as these articles, the internal temperature of the device may be taken as being the same as the articles. The temperature control system including an over-temperature alarm then responds to the temperature of the device which closely tracks the refrigerated articles. This approach may be useful when refrigerating articles of fairly uniform size and temperature constant characteristics. In view of the wide variety of items typically stored in a home refrigerator/freezer, a representative simulation device would be extremely difficult to design, and would add significantly to the materials and manufacturing costs. In addition, in such appliances storage space is at a premium and the simulation device would take up considerably more space within the freezer than does a simple thermistor type sensor.
In view of the shortcomings of these known approaches, it is desirable to provide an over-temperature warning system for a refrigerator/freezer appliance of sufficient sensitivity to reliably detect and alert the user to over-temperature conditions in the freezer resulting from abnormal system operation in timely fashion so as to enable the user to take appropriate preventive action to protect the refrigerated items from damage yet which does not respond to temporary over-temperature conditions such as typically result from the normal opening and closing of the freezer door and from automatic defrost cycles.
It is therefore an object of the present invention to provide an over-temperature warning system which alerts the user to an over-temperature condition when the sensed temperature exceeds a first relatively low reference temperature for a relatively long time or exceeds a second relatively high reference temperature for a relatively short time, thereby avoiding nuisance trips while alerting the user to abnormal conditions before damage to the refrigerated items has occurred.
It is a further object of the present invention to provide an over-temperature warning system of the aforementioned type which also generates a warning signal that either the lower reference temperature or the higher reference temperature has been exceeded for respective time periods of sufficient duration that damage could have already occurred and which signal continues pending user intervention even though the sensed temperature may have subsequently returned to within its normal operating range.